1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a wireless communication system for carrying out wireless communication while executing handovers.
2. Description of the Related Art
A wireless communication scheme called WiMAX (Worldwide Interoperability for Microwave Access) is being standardized by the IEEE (Institute of Electrical and Electronics Engineers). While conventional wireless LAN is primarily aimed for indoor use, WiMAX is a wireless communication standard adapted for a wider area such as a metropolitan area.
In order to enable fast handovers, WiMAX provides a function called FBSS (Fast Base Station Switching). With FBSS, each mobile station (MS) holds a Diversity Set identifying a plurality of base stations (BSs) of which the radio wave strengths in terms of C/I (CINR: Carrier-to-Interference plus Noise Ratio) are of a satisfactory level higher than a certain value, and previously establishes packet-based synchronization with each of the base stations included in the Diversity Set, to carry out a handover at high speed when necessary.
FIG. 13 illustrates a network wherein handover is executed according to FBSS. A WiMAX network 100 comprises a gateway 101 (GW), base stations 102 to 104 (BS1 to BS3), and a mobile station 105 (MS). The mobile station and the base stations have timers synchronized with one another and are capable of referring to the identical time.
Let us suppose here that the mobile station holds a Diversity Set identifying BS1, BS2 and BS3, and that before movement, the mobile station is exchanging packets with BS1 as an Anchor BS (the base station with which the mobile station is actually communicating, among those included in the Diversity Set).
S71: The mobile station exchanges packets with BS1, which is the Anchor BS and is included in the Diversity Set.
S71a: The mobile station previously establishes packet-based synchronization not only with BS1 but also with BS2 and BS3 included in the Diversity Set.
S72: The mobile station moves to the area of BS2.
S73: The mobile station and BS2, between which packets are to be exchanged after a handover, determine in advance the time for switching over to BS2, and the switching from BS1 to BS2 is executed exactly at the determined time to carry out the handover.
S74: BS2 acts as a new Anchor BS and communication is performed between the mobile station and BS2.
The FBSS operation described above is executed at the WiMAX layer (corresponding to Layer 2) and is standardized; however, nothing is stipulated as to the connectivity of the IP (Internet Protocol) layer higher in level than the WiMAX layer.
Meanwhile, Mobile IP (RFC 3344) is known as a scheme for implementing IP Mobility (technique whereby, even after the movement of a mobile station, IP packets can be exchanged by using the same IP address).
Predictive type fast handover Mobile IP is a scheme based on Mobile IP and enabling faster handovers, and Fast Handovers for Mobile IPv6 (RFC 4068: FMIPv6) and Mobile IPv4 Fast Handovers (draft-ietf-mip4-fmipv4-00.txt; in process of standardization; FMIPv4) are known as such scheme.
According to Predictive type fast handover Mobile IP, a mobile station acquires in advance information about an L3 (Layer 3) network (corresponding to IP) as the target of handover. During an L2 (Layer 2) handover, packets are forwarded to and buffered in the router of the target network.
On completion of the L2 handover, the mobile station transmits a packet transfer initiation message to the target network, whereupon the target network starts to transmit the packets to the mobile station, so that the mobile station is handed over at high speed without the need for L3 re-establishment.
The L3 re-establishment is unnecessary because the mobile station previously acquires information about the target L3 network. The L2 in the Predictive type fast handover Mobile IP corresponds to the aforementioned WiMAX layer.
As conventional techniques for controlling handovers among mobile and base stations, a technique has been proposed in which the mobile station measures a packet timing difference and sends the measured difference to the base station, and the base station corrects the phase of packets involving the timing difference, so as to establish synchronization at the time of handover (e.g., Unexamined Japanese Patent Publication No. 2000-69526 (paragraph nos. [0046] to [0051], FIG. 1)).
In WiMAX wireless networks, handovers at both Layers 2 and 3 can be executed by combining an FBSS-based fast handover scheme with a fast handover scheme used in Mobile IP, such as FMIPv6.
FIG. 14 shows a wireless network wherein FBSS and FMIPv6 are combined to carry out handovers. The wireless network 200 comprises a core network 210, access networks 220 and 230, and a mobile station 240 (MS).
The core network 210 includes a correspondent node 211 (CN) and a home agent 212 (HA). The access network 220 includes a previous access router 221 (PAR), a base station 222 (BS1), and a previous base station 223 (PBS). The access network 230 includes a new access router 231 (NAR), a base station 233 (BS2), and a new base station 232 (NBS).
The “previous base station” and the “new base station” are so termed because they act as such with respect to the mobile station, and each base station can function as both the previous and new base stations.
Also, the naming of the individual devices on the wireless network 200 follows the terminology according to the standard specification RFC 4068 for FMIPv6 (the terminology is basically identical with that of FMIPv4), where PBS/NBS corresponds to BS in WiMAX and PAR/NAR corresponds to GW in WiMAX.
When moving from the access network 220 to the access network 230, the mobile station makes a switching from PBS to NBS in the Diversity Set (Anchor BS is switched from PBS to NBS). With the movement of the mobile station, the route of exchanging packets changes as r1→r2→r3.
FIGS. 15 and 16 are sequence diagrams illustrating a handover executed in the wireless network 200, wherein Steps S82 through S86 are a preparatory process and Steps S87 through S97 are a handover process. Also, in the figures, WiMAX control messages for FBSS are enclosed in solid-line rectangles, and FMIPv6 control messages are enclosed in dashed-line rectangles.
S81: Packets delivered from the home agent are forwarded to the mobile station via the PAR and the PBS.
S82: MOB_MSHO_REQ (NBS) is a Diversity Set recommended BS request message. The mobile station sends this message to the PBS to inquire whether the NBS may be added to the Diversity Set.
S83: MOB_BSHO_RSP (NBS) is a Diversity Set recommended BS response message. Using this message, the PBS notifies the mobile station that the NBS can be added to the Diversity Set. By exchanging messages in Steps S82 and S83, the mobile station and the PBS negotiate a base station that can be added to the Diversity Set.
S84: MOB_HO_IND (NBS) is a Diversity Set update message. By means of this message, the mobile station finally decides the base station to be added to the Diversity Set, and then updates the Diversity Set.
S85, S86: RtSolPr (Router Solicitation for Proxy Advertisement) is an L3 network information request message, and PrRtAdv (Proxy Router Advertisement) is an L3 network information advertisement message. The mobile station sends RtSolPr to the PAR, and the PAR returns PrRtAdv to the mobile station.
Specifically, after the update of the Diversity Set is completed, L3 network information (network prefix, router IP address, CoA (Care of Address), etc.) about the new access router (AR) to which the base station added to the Diversity Set belongs is acquired by means of RtSolPr and PrRtAdv.
S87: FBU (Fast Binding Update) is an L3 network handover initiation message. By means of the FBU message, the mobile station notifies the PAR of the initiation of a handover, namely, that the mobile station is moving to the network of the NAR.
S88, S89: HI (Handover Initiate) is an inter-L3 router handover initiation message, and HAck (Handover Acknowledge) is an inter-L3 router handover response message. The PAR sends the HI message to the NAR to notify same that the handover is to be executed, and the NAR returns the HAck message by way of a response.
S90, S91: After the handover initiation messages are exchanged between the routers, the PAR stops transmitting packets to the mobile station and forwards, to the NAR, the packets to be transmitted to the mobile station, so that the packets are buffered in the NAR.
S92: FBack (Fast Binding Acknowledgement) is an L3 network handover response message, which is responsive to the message received in Step S87.
S93: MOB_HO_IND (NBS) is an L2 handover initiation message. Although the name of this message is identical with that of the message sent in Step S84, the PBS can distinguish between the two messages because the parameters included therein are different. On sending out this WiMAX control message, the mobile station switches the Anchor BS from the PBS to the NBS.
S94: Anchor_BS_Switch_IE is an L2 switching time designation message. This message notifies the mobile station of the designated timing for carrying out L2 switching.
S95: The mobile station carries out L2 link switching at the designated timing.
S96: FNA (Fast Neighbor Advertisement) is an L3 packet transmission initiation message. Since the L2 connection is completed in Step S95, the FNA message is sent to the NAR to instruct same to establish L3 connection. The FNA message signifies that the handover at L2 is completed and also that transmission of packets from the NAR to the mobile station should be started.
S97: On receiving the FNA message, the NAR transmits the buffered packets to the mobile station via the NBS.
S98, S99: BU (Binding Update) is an L3 packet route switching message. When this message is received, the home agent switches the target of packet transfer from the PAR to the NAR (the Anchor router is switched from the PAR to the NAR).
During the aforementioned process, packets (actual traffic packet data exchanged between the home agent and the mobile station) flow in the manner described below. From Step S81 to Step S89, packets flow via the route r1 shown in FIG. 14, from Step S90 to Step S98, packets flow via the route r2, and after Step S99, packets flow via the route r3.
In the handover process involving two layers, the link of the upper Layer 3 (FMIPv6) and then the link of the lower Layer 2 (WiMAX) are disconnected in order. When establishing links, on the other hand, the Layer 2 link and then the Layer 3 link are established in order.
The control flow described above permits the handover process involving two layers to be carried out using the combination of FBSS and FMIPv6. However, the handover process is associated with the problems explained below.
First, there occurs a time lag between the L2 switching in Step S95 and the arrival of packets at the mobile station. As soon as the L2 switching is performed, the mobile station is ready to receive packets. According to the above control flow, however, after the L2 switching, the mobile station sends an FNA message requesting packet transmission, and on receiving the message, the NAR forwards the packets to the mobile station. Consequently, a time lag occurs between the L2 switching and the packet reception, and this hinders speeding up the handover process.
Further, in the case of the handover involving two layers, Layer 2 control messages and Layer 3 control messages are exchanged separately, giving rise to a problem of increased traffic and consequent low efficiency.
Specifically, as seen from Steps S82 to S86, the control messages for FMIPv6 and the WiMAX control messages for FBSS are exchanged separately, and therefore, the number of times the messages are exchanged (the number of messages exchanged) is large, requiring extra time for the handover.
The handover preparatory process has no direct connection with the time required for the handover process itself. In FBSS, however, the Diversity Set is updated during the preparatory process, and the base stations registered in the Diversity Set are switched thereafter. Accordingly, if the update of the Diversity Set is delayed in the preparatory process, the initiation of the handover process is also delayed.